Wireless communications systems are used in a variety of telecommunications systems, television, radio and other media systems, data communication networks, and other systems to convey information between remote points using wireless transmitters and wireless receivers. A transmitter is an electronic device which, usually with the aid of an antenna, propagates an electromagnetic signal such as radio, television, or other telecommunications. Transmitters often include signal amplifiers which receive a radio-frequency or other signal, amplify the signal by a predetermined gain, and communicate the amplified signal. On the other hand, a receiver is an electronic device which, also usually with the aid of an antenna, receives and processes a wireless electromagnetic signal. In certain instances, a transmitter and receiver may be combined into a single device called a transceiver.
Wireless communication systems are commonly susceptible to spurious emissions. Spurious emissions are undesired signals that appear in electronic equipment, often at a harmonic frequency of desired signals. In transmitters and transmit paths of transceivers, a type of spurious emission known as 4 times baseband or “4×BB” spurious emission may occur. 4×BB emission often occurs at interfaces among baseband filters, upconversion mixers, and variable gain amplifiers in a transmit path, and causes two distortion terms on either side of a carrier frequency for a radio-frequency signal. The 4×BB peaks often occur at a frequency that is higher or lower than the carrier frequency in an amount equal to four times the baseband signal offset frequency. 4×BB spurious emission may also be known as C-IM3 (3rd order distortion) and folded C-IM3 (5th order distortion). In some cases, 4×BB spurious emissions may occur at magnitudes that may violate relevant wireless communication standards (e.g., 3rd Generation Partnership Project or “3GPP”) or cause operational problems in wireless communication devices.
4×BB spurious emissions are seen to be created by a wireless communication device's nonlinear effects on baseband signals and carrier signals, hence, 4×BB emissions may be cancelled out by digitally pre-distorting the digital baseband signal with the opposite phase of the predicted emissions.
In order to limit emission and noise, analog filters are deployed along with digital filters in transmitters. The 3 decibel (dB) bandwidth of the composite filters is often chosen to be comparable to transmitted signal bandwidth. Due to frequency hopping in a 3GPP Long-Term Evolution (LTE) system, for any digital pre-distortion to work effectively, the composite filter needs to have a flat amplitude response from direct current to the frequency of the 4×BB distortion terms. In addition, it is desirable that the composite filter have a linear phase response. A linear phase response may be achieved by applying a pre-emphasis filter to the predicted emissions of the digital pre-distortion.
Effective pre-emphasis and pre-distortion may be complicated by the numerous factors, including:                process variations that may cause an analog filter response to vary;        temperature variations may cause physical characteristics of analog filters to vary, thus leading to variance of analog filter response;        temperature variations may cause impedance changes at an interface between a switching mixer and a variable gain array input, as does the nonlinear effects causing 4×BB emissions;        a power control system may alter variable gain amplifier gain on LTE time slot boundaries, thus altering loading seen by a mixer; and        a power control system may alter signal strength to a mixer.The above factors may require different pre-distortion and pre-emphasis filter designs to maximize performance.        
The foregoing section merely provides a background to the embodiments of the present disclosure, and no portion thereof shall be construed as an admission of prior art.